Internet Public Library http://www.ipl.org/div/kidspace/
Kidspace research area with links to many online resources including databases and encyclopedias. Encyclopedia.com
http://www.encyclopedia.com/
InfoPlease
http://www.infoplease.com/encyclopdict.html
The Library of Congress
http://www.loc.gov/index.html
Contains numerous databases of images and primary source documents and information of people, places, and events from US history. The "Kids and Families" section is a good starting point for younger students.